The present invention relates to a plasma spraying process and apparatus, which more particularly makes it possible to form deposits on substrates having a poor resistance to high temperatures. It also makes it possible to deposit chemically active substances or substances with a strong affinity for oxygen.
Plasma spraying is well known and widely used for producing coatings, e.g. on composite materials, such as resins or plastics which have to be coated with metallic or ceramic coatings. This process is also used for forming protective coatings in the mechanical field (aeronautical or automobile alloys) or in the nuclear field (production of absorption barriers).
Initially this operation was performed in air, which made it impossible to deposit certain materials which react with oxygen, such as borides and carbides. Deposition is also not possible on supports, which are themselves reactive, such as aluminium, magnesium or their alloys. Consideration was therefore given to the spraying operation being carried out either in a neutral atmosphere, e.g. in chambers under argon, or in an enclosure under a partial vacuum the residual pressure being approximately 50 to 200 millibars. This process makes it possible to operate at high temperatures of approximately 600.degree. to 700.degree. C. and to coat superalloys, whilst facilitating the diffusion of the product sprayed into the basic metal. However, this process cannot always be used when the substrate is a material having poor high temperature resistance. This is the case with certain composite materials, such as resin-bonded silica fibres, which become unstable beyond 120.degree. C. and with which plasma spraying is difficult to envisage, because this operation leads to much higher temperatures. In particular, this method does not make it possible to produce very thick deposits, due to cracking phenomena.